This invention relates to an apparatus for nondestructively measuring various characteristics of a semiconductor wafer with a junction such as the lifetime of a carrier, the cut-off frequency of the junction and the like by use of a photovoltaic effect.
First, measurement of the carrier lifetime will be described.
Recently, planar type solid state circuit devices have become predominant among solid state circuit devices in general and more definitely, these devices are formed on an a thick semiconductor wafer which is typically about 400.mu. thick. Needless to say, since the solid circuit devices make use of the transport phenomenon of the current carrier (hereinafter referred to as the "carrier") inside the semiconductor, the lifetime of the carrier inside the wafer exerts significant influences upon the characteristics of the device in the solid state device manufacturing.
To improve the yield of a large number of devices cut out from a single wafer, it is an essential condition that the carrier lifetime inside the wafer be uniform irrespective of the relative position of the wafer positions from which the device is taken.
In solar cells, on the other hand, there is a requirement that the carrier lifetime be long over a wide area so as to improve the conversion efficiency of the solar energy.
It is therefore of the utmost importance to measure and evaluate in advance the carrier lifetime inside the wafer as the substrate not only for solid state circuit devices but also for solar cells.
To the object described above, an apparatus has been commercially available which excites the carrier inside the semiconductor wafer and measures the decay of the carrier by use of microwaves. The apparatus has been effectively used as a lifetime measurement equipment of the wafers before a junction or junctions are formed.
However, in order to operate the solid state device as an active device, whether the solid state device may be the solid state circuit device or the solar cell, a junction exemplified by a p-n junction must be formed on the wafer and hence, the solid state device is mostly subjected to the thermal process of a temperature of about 800.degree. C. to about 1,000.degree. C., including the ion implantation process and its subsequent annealing process. As is well known, after the wafer is subjected to the thermal process, the carrier lifetime drops more than before the thermal process because oxygen and other trace impurities contained in the wafer precipitate and diffuse.
Accordingly, the carrier lifetime measured for the bulk wafer does not eventually represent the finished solid state device. It is therefore necessary to measure and evaluate by all means the carrier lifetime after the junction is formed. Quite naturally, however, the conventional measuring method of the carrier lifetime by photo-excitation microwave measurement can not be used any longer. The reason for this is that the resulting junction-forming layer masks the microwave response and hence, detection is no longer possible using the microwave technique.
Accordingly, several methods have been proposed in the past to measure the carrier lifetime of the wafer substrate after the junction is formed. Among these methods, the photocurrent method which is most analogous to the present invention will be described.
FIG. 1A shows the principle of the photocurrent method. The drawing shows the case of a p-n junction formed by a p-type Si substrate (wafer) 1 and an n-type layer 2 disposed on the substrate, by way of example. When the photo beam 3 is radiated to this junction, a photocurrent develops through the junction as is well known in the art. The photocurrent excited by the photo beam 3 can be taken out from the junction by disposing metallic electrodes 4 and 4' on the wafer substrate 1 and on the n-type layer 2, respectively. The current can be accurately read by a meter 6 after it is amplified by an amplifier 5.
If the photo beam is pulsated and the chopping frequency is changed so as to read the photocurrent, the photocurrent I.sub.ph changes, as illustrated in FIG. 1B. For an angular frequency .omega.=2.pi..times. (pulse frequency), the photocurrent I.sub.ph remains substantially constant so long as the angular frequency .omega. is small. But when the angular frequency .omega. exceeds .omega..sub.0 , for example, the photocurrent decreases in proportion to .omega..sup.-1/2. The bending point of this photocurrent I.sub.ph can be diagrammatically obtained simply.
If the assumption that almost all the photocurrent occurs on the substrate 1 is established, the relation between .omega..sub.0 and the carrier lifetime .tau. can be easily expressed by the following formula: EQU .omega..sub.0 .tau.=1 (1)
If .omega..sub.0 is obtained from FIG. 1B, the carrier lifetime .tau. can be easily obtained by calculation from the formula (1).
If the junction has in advance the metallic electrodes 4 and 4' as in the case of the solar cell, the photocurrent method is extremely convenient.
However, even the solar cell does not have the metallic electrodes immediately after the junction is formed and the conventional photocurrent method can not be employed in such a case. In the case of the solid circuit device process, the junction is formed at the initial stage of the process and the electrodes can not be formed at the junction in order to prevent contamination that occurs in the subsequent several steps. Accordingly, if the conventional photocurrent method is used as such, it means discard of the subsequent steps (that is, to discard the products as the defective products). In this sense, the photocurrent method becomes a kind of destructive inspection method.
Besides the p-n junction, other types of junctions include a Schottky barrier junction between the metal and Si and a p.sup.+ -p type junction or a so-called high-low junction. In either case, the inspection becomes descructive inspection if the measuring metallic electrodes are formed.
Still another kind of junction is a so-called "Field Induced Junction" exemplified by SiO.sub.2 -p type Si. In this junction, the photocurrent generated at the junction can not be detected even if the metallic electrodes can be formed on the SiO.sub.2 film, unless the SiO.sub.2 film is as thin as about 50 .ANG.. This is the case since SiO.sub.2 is an insulation material and does not generally allow the passage of a current. The conventional photocurrent method can not be applied completely to this kind of junction. As is well known from an FET transistor, however, the field induced junction has gained an extremely wide application nowadays in the same way as the p-n junction. Hence, any measuring method of semiconductor characteristics is fatal unless the method can also be applied to the field induced junction.
Next, measurement of the cut-off frequency will be described.
As is well known, a resistance component (R.sub.j) and a capacitance component (C.sub.j) exist in parallel with each other in the p-n junction of the semiconductor described above. When a predetermined a.c. current i is applied to this p-n junction, the terminal voltage v of the p-n junction decays in inverse proportion to the frequency f above a predetermined frequency f.sub.c. This can be numerically expressed by the following formula (2): ##EQU1##
The frequency f at which the terminal voltage v of the p-n junction starts decaying is defined as the cut-off frequency f.sub.c and is used as a numeric value representative of the characteristics of the p-n junction. This frequency is given by the following equation (3): ##EQU2##
It has been a customary practice to measure the characteristics by applying the current i from outside, using the electrodes formed on the finished p-n junction in accordance with the method described above.
However, the solid state device industry now requires to know the cut-off frequency of the p-n junction before the the electrodes are formed on the p-n junction. This is quite natural because the quality of the solid state devices must be determined at a stage as early as possible in order to improve the yield.
In the semiconductor device manufacturing process, the metallic electrodes for measurement or the like must not be brought into contact with the portion of the solid state device portion such as the p-n junction from the aspect of prevention of breakdown, contamination and the like, as described already. Hence, this requirement can not be satisfied unless the method is a nondestructive method.